1. Field of the Invention
The present invention relates to the manufacture of bonded non-woven fibrous mats, particularly comprised of glass fibres or mineral wool.
2. Description of the Art
Such mats can be made by forming an air-laid mat, for example, by subjecting streams of molten glass or rock to the action of hot gases in the form of steam/air jets and to mechanical attenuation, prior to collection on a foraminous receptor surface in a forming hood. They can also be made from continuous filaments or strands, either as such or after chopping into staple fibre form prior to deposition on a receptor surface. A binder is sprayed onto the mat during or after formation and is thereafter cured in situ by the action of heat to form a bonded mat. Phenolic and polyester resins are commonly used in such processes, although other binders may also be used. Hot air and/or radiant heat are generally used to set the binder, for example by curing the resin.
The binder spraying step tends to be rather haphazard. It frequently results in excessive local concentrations of binder, losses of binder into the atmosphere and consequent pollution problems. Because of the high temperature of the newly-formed fibres an the presence of the hot gases used to attenuate them, the binder has to be sprayed at a high water content (for example, 80- 90% water, 10- 20% binder), although in practice most of this water is lost by evaporation together with a significant proportion of the binder. The variations in the binder content throughout the mat may be considerable, for example, .+-. 5% relative to a target binder content in the finished product of 10% by weight, which means that in general the level of binder addition must be appreciably higher than would be necessary if the distribution were sensibly even.
It has been proposed to make bonded non-woven fibrous mats by a process including the steps of forming a foam from a suspension/solution of the binder and impregnating a non-woven fibrous mat with the foam. In such a process the impregnation step is carried out by applying the foam as a layer on one face of the mat, the foam being squeezed into the mat by a roller, doctor blade or an endless belt, assisted by the application of suction at the opposite face of the mat. After impregnation, the binder may be caused to set in situ, for example, by heating, as mentioned above. However, the application of this process to glass fibre and/or mineral wool mats has proved difficult due to the very high porosity of such mats and the tendency of the binder to migrate on drying/curing. Furthermore, the application of a foamed binder to a newly-formed mat of hot fibres on a production line can result in premature and/or uneven setting/curing of the binder, before proper impregnation of the mat has been accomplished.
According to the present invention, a bonded non-woven glass fibre or mineral wool mat is made by a process including the steps of forming foam from an aqueous solution or suspension of a binder, impregnating a non-woven mat with the foam and then selectively removing a major portion of the water from the mat prior to setting/curing the binder.
Preferably the water is removed by the application of high frequency dielectric heating.
Preferably, the amount of foamed binder applied to the mat is in excess of that required to give the desired binder solids content in the final dry product, the extent of the excess being of the order of 50% or more by weight, and the surplus being drawn through the impregnated mat prior to selectively removing the water. Advantageously, the surplus is thereafter collected and re-circulated. Thermosettable resins are especially preferred as binders.
It has been found that the application of binder as an aqueous foam, particularly when applied in the excess just referred to gives significantly more uniform impregnation, typically of the order of .+-. 1% of the target value as opposed to the relatively high variability experienced with prior art processes. This means that one can use less binder for a particular product density while maintaining substantially the same physical properties such as strength and rigidity. It also means that there can be a significant economy in binder consumption by virtue of the elimination of the conventional process step of spraying the binder into the forming hood mentioned earlier. Losses due to evaporation/binder carry-over into the mat-forming hot gas streams through the foraminous receptor surface and the hot fibres are typically of the order of 15-30% and these losses can be eliminated for all practical purposes. With more efficient binder utilization there may also be enhanced fire resistance due to the reduced total organic content of the final product. The water content of the foam is determined by a number of factors such as the temperature of the mat and the final target binder content in the product. Because the foam contains water it can safely be applied even to a hot mat on a production line, provided that the water content is high enough to cool the mat and at the same time prevent significant curing/setting of the binder. However, it should be noted that even if the foam is applied to a hot mat, the water content of the impregnated mat (prior to selective removal of the major portion of the water) will be considerably higher than the water content of a similar bonded mat made by the traditional forming hood spraying process. For example, the latter process gives typical water contents of 4- 6% (by weight), whereas the process of the present invention gives 10- 30% water content. The use of a high water content and an excess of foam contributes not only to the uniformity of impregnation but also to maintaining that uniformity. The use of high frequency heating is particularly advantageous in this context since it enables the water to be removed from the whole thickness rather than from a single surface exposed to conventional heating. Effectively, high frequency heating dries the mat from the inside outwards to the surfaces. It also enables the mat to be dried to a controlled water content.
It has also been observed that high frequency heating causes a significant increase in the bulk of the mat, i.e. an increase in the thickness of the mat on selective removal of the water. The increase can be as much as three or four times the wet thickness of the mat immediately after impregnation. Although conventional drying methods do produce an increase in bulk, it has been observed that the increase caused by high frequency heating is usually greater. One result of this is that relatively low final dry densities of the order of 16 Kg/m.sup.3 are possible. However, it is also possible to produce final densities of the order of 320 or more Kg/m.sup.3 by compacting the dry mat prior to curing/setting the binder. The process of the present invention therefore exhibits a considerable degree of flexibility and enables the production of products over a wide range of final, dry density.
The process of the present invention is especially applicable to the production of mats of fibrous thermal insulation made from glass fibres, mineral wool or rockwool, where the binder is used to impart a degree of resilience and cohesion to the product without significantly reducing its porosity. The invention includes such products when made by a process according to the invention.
However, high frequency heating was previously thought impracticable in glass fibre/mineral wool manufacture because conventional binder application processes produce clots of binder in the mat and these react adversely to high frequency heating.
Using the process of the invention, it is also possible to impregnate a mat off the production line, which in some circumstances adds flexibility to the overall manufacturing process. In this particular case, the water content of the binder need not be as high as would be necessary for on-line application, because the mat would not normally need cooling. The binder and the water may be individually metered to give the mixture which is foamed. This is preferred because it makes for maximum control of the binder solids present in the final dry product and makes changes in binder solids very easy to effect.
While the high frequency heating is preferably applied only for as long as is necessary to selectively remove a major portion of the water, conventional heating being then used to set/cure the resin, it is also possible to use high frequency heating to set/cure the binder, before, during or after conferring a desired configuration on the mat. For example, a 6 to 12 second treatment can substantially dry a typical foam impregnated mat without significant effect on the binder. Where the binder is a thermosettable resin, a 20 second treatment would at least initiate curing of the resin. However, at production line speeds, such a long treatment time may be impracticable, or unduly expensive, or both.
Hitherto, the aplication of binder after mat formation has been difficult, if not entirely impracticable because of the desired bulky foraminous nature of the product. Uniform impregnation is not achieved by spraying binder onto the faces of an already-formed mat and the use of a liquid binder tends to destroy the desired bulk. For these reasons, shaped products such as lengths of tubular pipe insulation have always been made from mat impregnated with a settable/curable binder during production. Because it is no longer necessary to impregnate with binder during mat formation, it is now possible to make and store completely unimpregnated mat for future use, without having to worry about binder shelf life and/or variations in binder properties from batch to batch. This unimpregnated mat can be subsequently impregnated, dried and shaped into such products as lengths of tubular pipe insulation, prior to setting/curing the binder in the usual way. The invention includes products made in this way from unimpregnated mat. The present invention thus enables impregnation to be carried out at the most convenient time or location, thereby giving an essentially two-stage process in which mat formation and binder impregnation can be separated by any desired interval, with or without the optional step of also forming a shaped product from the impregnated mat.
A further advantage of high frequency heating lies in the fact that selective removal of water can be accomplished without producing significant binder fumes. The output airstream from the high frequency heater may even be vented to atmosphere without the usual effluent problems.
Advantageously, the aqueous binder solution/suspension contains a minor amount of a surfactant to facilitate foaming; the exact level of surfactant addition and the solids content (dilution) of a particular binder being a matter for experiment in order to arrive at a desired binder content in the final product.
A further advantage of the process of the invention is that excess foamed binder and any fibres therein can be re-circulated because the binder does not have to be sprayed and because it is still neither set nor cured.
The invention also includes apparatus for carrying out the process of the invention, said apparatus including a foam generator, means for spreading an aqueous foam made thereby onto one face of a non-woven mat, means operable to urge the foam into the mat together with heating means operable to selectively remove a major portion of the water from the impregnated mat. The heating means is preferably a high frequency heater. Preferably the apparatus further includes means for collecting and recirculating excess foam from the mat during and/or after impregnation thereof.
The means for urging the foam into the mat preferably includes both a suction device operative on the opposite face of the mat to that onto which the foam is spread and means for pressing the foam into the mat towards said suction device. Because glass/mineral wool mats are very porous, the means for pressing the foam into the mat should preferably seal against at least that face of the mat to which the foam is applied. Otherwise, the suction device may simply draw air through any relatively thin or more permeably parts of the mat without thoroughly impregnating them with the foam. Particularly preferred apparatus for this purpose comprises a foraminous roller containing at least one suction head and an impermeable endless flexible belt, one run of which presses against the mat as it passes over the roller. The roller is preferably in the form of a drum with peripheral, radially-extending flanges because the use of a drum and particularly one with sidewalls has been found to give a better seal between the flexible belt and the drum, thereby utilising the suction more efficiently. Where a high frequency heater is used to selectively remove water from the impregnated mat, it is preferably a radio frequency heater. Typically, such heaters operate at a frequency of approximately 27 mHz.